1. Field of the Invention
The present invention relates to an image forming apparatus, a color image forming apparatus, a scanning unit, and a scanning lens. The present invention particularly relates to an image forming apparatus and a color image forming apparatus having a resinous scanning lens through which a light beam deflected in a main scanning direction passes; a scanning unit for emitting a light beam and scanning the light beam over a scanning target; and a scanning lens employed in the scanning unit.
2. Description of Related Art
In a laser printer or other electrophotographic image forming apparatus, a light source such as a semiconductor laser emits a light beam that is deflected by a polygon mirror or other deflector onto a photosensitive member, the surface of which has a uniform charge, so as to form an electrostatic latent image on the surface of the photosensitive member by scanning the light beam thereon. The latent image is subsequently developed into a visible image with toner, and the toner image is transferred onto a recording medium, such as a sheet of paper, to form an image on the recording medium.
This kind of conventional image forming apparatus has a scanning unit for irradiating the light beam on the surface of the photosensitive member. The scanning unit is often configured of a light-emitting unit such as a semiconductor laser for emitting the light beam, a deflector such as a polygon mirror for scanningly deflecting the light beam in a main scanning direction, a first scanning lens such as an fθ lens for converging the light beam in the main scanning direction, and a second scanning lens such as a cylindrical lens for converging the light beam in a sub-scanning direction perpendicular to the main scanning direction. These components may be mounted on a unit frame formed of synthetic resin, for example.
While scanning lenses such as the fθ lens and the cylindrical lens were conventionally formed of glass, many of these lenses are now being formed of a synthetic resin material due to the low manufacturing cost and the ease with which such lenses can be mass produced in metallic molds, even lenses with complex curvatures.
Lenses made of a synthetic resin material are often not produced in the ideal shape defined by the molds and may be deform slightly during production due to a non-uniform temperature distribution in the mold, uneven cooling, and the like. Cylindrical lenses and other scanning lenses formed of synthetic resin are particularly susceptible to warping in the longitudinal direction. A technique designed to correct this warpage has been disclosed in Japanese patent-application publication No. 2001-91879.